1. Road Design

2. Design standard
Design standards refers to strategic decisions regarding the geometric standard to which the road is build
These decisions usually made at planning stage and primarily affected by capacity and economic efficiency considerations
Safety should also be a consideration in planning phase

3. Road standard
As traffic flow increases it becomes necessary or economical to design and build roads to higher economic standard
The higher the economic standard, the safer the road
Safety benefits of higher geometric standards is one of the economic factors that should be taken to account in deciding appropriate design standard

4. Road standard
Investigating safety benefits of road improvement show the following crash reduction significant at 10 percent level or better for various types of projects
Rural town bypasses : 32% reduction
Rural road duplication : 29% reduction
Urban intersection grade separation : 57% percent
Other rural projects : 28% reduction

5. Road standard The highest geometric of road is one with
High design speed
Full access control
Forgiving roadsides
Entry and exit at grade-separated interchanges
Opposing direction of traffic separated by a median
This type of road is variously referred to as freeway, motorway, autobahn, and sometimes expressway

6. Road standard Safety improves dramatically with design standard
Freeways are much safer per mile of travel than other roads
Freeways with higher standards are
at lest 4 times safer than other roads
20 times safer than other arterial rods
2 times safer than freeways with lower standards

7. Access control
Access control reduces or eliminates the events which the driver must respond
Access control has been described as the most important single design factor ever developed for crash reduction
Part of safety advantages of freeways stem from control of access
Controlling access on existing road through the use of frontage roads

8. Access control (continued)
Crash rates increases rapidly with the density of access driveways
In one study the difference between a low development (fewer than 30 access per mile) and a high level of development was to more than double the number of driveway crashes
Several studies indicated that this is a rural problem as well as an urban problem in US

9. Median Medians are of several types
Wide median without a physical barrier
Provide space for the driver to regain control and provide room for turning lanes
Narrow median with a physical barrier
Steel guard fence or shaped concrete barrier
Discouraging inappropriate pedestrian crossing
Narrow median without a physical barrier
Separating opposing direction of traffic
Provide opportunity for pedestrians to cross the road in two stages

10. Median (continued)
A study by National Association of Australian state road compared crash rate for roads with different median types and compared with undivided roads
Narrow painted median
30% crash reduction
Narrow raised median
48% crash reduction
Wide median
54% crash reduction

11. Median (continued)
In urban areas, medians should be wide enough to protect a turning or crossing vehicles
FHWA in 1982 found that with a 30 feet wide median, between percent of vehicle encroaching on the median do not reach the other side of roadway
Narrower medians with physical barrier typically have higher crash rate but lower crash severity

12. Median (continued)
A British study in 1980 found that installing a steel guard fence in the median of divided rural roads led to
15% reduction in fatal crashes
14% increase in non-injury crashes
The median slope on wide medians can influence crashes
Zegeer and Council 1992 suggest that
a maximum slope of 6:1 is desirable on wide medians
Slopes of 4:1 or steeper are associated with rollovers

13. Cross section elements (continued)
Lane width
11-12 ft lanes have been shown to have the lowest crash rate
10 ft lanes have been shown to contribute to multi-vehicle crashes
A number of studies have shown the safety advantages of widening narrow lanes
An American study reveal 22% crash reduction on rural roads by widening 9 ft lanes to 11 ft and 10 ft lanes to 12 ft

14. Cross section elements (continued)
Widening lane width affected opposite direction and run-off-road crashes by following percentages
1 ft lane widening : 12% crash reduction
2 ft lane widening : 23% crash reduction
3 ft lane widening : 32% crash reduction
4 ft lane widening : 40% crash reduction
Study shows that there is little if any benefit in increasing lane width beyond 12 ft, except where there is a large volume of trucks, where lanes of 13 ft may be appropriate

15. Cross section elements (continued)
Wider lanes may be counter-productive, since they encourage unsafe maneuvers such as overtaking along center line in the face of oncoming traffic
Striping the road with three lanes
Better from safety and user service viewpoints
Clearly define overtaking lane n one direction
Much cheaper, since overtaking lane need not to be provided over full length of the road
Overtaking lane in about 10% of the road will accommodate the task

16. Cross section elements (continued)
Shoulder width
There is some evidence that crash rates reduce as shoulder width increases
An American study 1981showed a 21% reduction in total crashes when a road with no shoulders had shoulders of 3-9 feet
The study suggest that for the roads currently without shoulders, the optimum shoulder width to be provided is 5 feet
A Swedish study showed decrease in crashes with increase shoulder width from 0 to 7 ft, and a little additional benefit was obtained for shoulder width above 8 ft

17. Cross section elements (continued)
Lane and shoulder width
The effects of lane and shoulder width are not independent
Widening lanes from 9 to 12 ft without shoulder improvements reduce crashes by 32%
The greatest gain comes from the combination of lane and shoulder improvements
Widening a highway from 9 to 12 ft lanes and 0 to 6 ft shoulders, reduced crashes by about 60%

18. Cross section elements (continued)
Surface cross slope
Drainage is an essential part of any road
6 mm film of water can reduce friction coefficient to near zero, thus making braking and turning almost impossible
Most wet weather crashes occur on low skid resistance pavements
Dunlap 1978 found that thickness of a water film on large radius curves can be almost twice that on a crowned straight section of road with the same cross slope

19. Sight distance
A Swedish study 1990 found that in most cases crash rate decreases with increasing average sight distance especially single vehicle crashes at night
McBean 1982 found that on rural roads in Britain, sight distances shorter than 700 ft were more likely to be found at crash through their association with horizontal curves

20. Sight distance (continued)
A study reported by TRB, went on to develop a model to assist in determining the cost-effectiveness of lengthening a vertical curve to increase sight distance over a crest
It is cost-effective when the design speed is more than 20 mph below operating speed in the area
Traffic flows exceed 1500 vpd
High volume intersection
Sharp curve
Steep downgrade
Lane drop

21. Sight distance (continued)
Improving sight distance on horizontal curves has been found to be highly cost-effective
If involves low cost treatments
Clearing vegetation
Clearing Minor obstructions
Significant truck volume present
Since larger and heavier trucks have poorer braking performance despite increased eye height, it must be compensated by greater sight distance

22. Horizontal and vertical alignment
Average crash rate for curved road segment is three times that of straight segment
Average run-off-road crash rates is four times higher in curved segment than tangent
Studies suggested that horizontal curves on rural roads should not be less than about 2000 ft radius
A significantly higher crash rate can be expected on curves with radius below 1500 feet

23. Horizontal and vertical alignment (continued)
From safety viewpoint, the important issue is consideration of this factor in a consistent fashion with other design parameter along the stretch of road
Curve flattening is expensive and is only cost-effective under certain condition
Other treatments for safety problems at horizontal curve include
Physical rehabilitation and partial reconstruction
Removal of roadside hazards
Trees
Utility poles

24. Horizontal and vertical alignment (continued)
Flattening the side slope
Resurfacing the roadway to improve skid resistance
Increasing the superelevation
Paving the shoulders
Eliminating pavement age drops
Low cost treatments will include
Upgrading the pavement edge lines and centerlines
Adding raised reflective pavement markers
Providing curve alignment markers

25. Horizontal and vertical alignment (continued)
Upgrading the advance warning
Grades should ideally not exceed 6%, with a lower value 4% when there is a high proportion of trucks using the road
The worst situation happens when some road feature such as sharp curves, steep grades and intersections come together or in close proximity to each other
Horizontal curves should utilize plan transitions to connect the straight with circular arc, particularly on the roads with high proportion of trucks

26. Bridges, structures and culverts
Bridges and culverts can be significant in terms of in their involvement in run-off-road crashes
For new bridges, Mak 1987 has recommended that the bridges
should be 6 ft wider than traveled way, 3 ft shoulders should carried across the bridge
On roads with high traffic flows, full width shoulders may be carried across the bridge
Overpasses should have bridge piers which are designed for impact loading, ideally there should be no pier at the edge of the roadway

27. Overtaking Overtaking associated with crashes on rural two-lane roads
If there is guard fencing alongside the road on the bridge approaches, this will design to yield on impact
There is a need for transition in stiffness of the guard fencing adjacent to the bridge end post, and the guard fence should be rigidly attached to the end post
Overtaking associated with crashes on rural two-lane roads
Overtaking lanes provide significant safety benefits
In Australia, the recommendations for a two-lane rural road with design speed of 60 mph is to have

28. Truck escape ramps
A minimum length of overtaking lane of about 2000 ft and maximum of 4000 ft
The total taper length of 800 ft
Installation of an escape ramp is one of the few safety treatments designed for trucks to reduce the hazard of a runaway truck on a downgrade
There are six different types of design of truck escape ramp
Sand pile
Gravity ramp
Ascending-grade arrester bed

29. Truck escape ramps (continued)
Descending-grade arresting bed
Horizontal-grade arrester bed
Roadside arrester bed
FHWA quote a study in Colorado involving before and after study of the effectiveness of construction of truck escape ramps
The most successful ramp showed a 400% reduction in crashes and benefit-cost ratio of 10:1